1. Field of the Invention
The present invention relates to, for example, a motor-operated valve used for controlling a flow rate of a coolant in an air conditioner.
2. Description of the Related Art
FIG. 14 is a longitudinal sectional view showing an example of a conventional motor-operated valve disclosed in Japanese Patent Laid-Open Publication No. 2000-346226 (Patent Document 1). A motor-operated valve 100 includes a valve main body 120 having a valve chamber 121 and a valve seat 122 formed in the valve chamber 121, a valve body 123 that comes into contact with and moves away from the valve seat 122 to open and close an opening of the valve seat 122, and a can 140 fixed to the valve main body 120. A rotor 130 is built in the can 140. A stator 142 that drives to rotate the rotor 130 is externally fit over the can 140. A pipe 120a extending downward from the bottom surface of the valve main body 120 and a pipe 120b expending in the horizontal direction from the side of the valve main body 120 are provided in the valve main body 120. A coolant is led into the valve chamber 121 and the coolant in the valve chamber 121 is led out to the outside by these pipes 120a and 120b. 
The can 140 is formed of nonmagnetic metal and assumes a topped cylindrical shape. The can 140 is fixed to a collar-like plate 141, which is fixed in an upper part of the valve main body 120, by welding or the like. The inside of the can 140 is kept in a hermetically sealed state. The stator 142 includes a stator main body 143 and a jacket section 144 made of resin that covers the outer side of the stator body 143. The stator main body 143 includes a yoke 151 made of a magnetic material and upper and lower stator coils 153 wound around the yoke 151 via a bobbin 152. The jacket section 144 has a fitting hole 144a that is externally fit over the can 140.
The valve body 123 including a needle valve is formed at the lower end of a valve shaft 124. The rotation of the rotor 130 is converted into actions of the valve body 123 coming into contact with and moving away from the valve seat 122 by a driving mechanism. This driving mechanism is a feed screw mechanism including a tubular guide bush 126 that is fixed in the valve main body 120 to project in the direction of the rotor 130 and in which a fixed screw section 125 is formed and a valve shaft holder 132 having a moving screw section 131 that screws in the fixed screw section 125. The fixed screw section 125 is formed as a male screw formed in the outer periphery of the guide bush 126. The moving screw section 131 is formed as a female screw formed in the inner periphery of the valve shaft holder 132.
The valve shaft holder 132 is located on the outer side of the guide bush 126 and assumes a topped cylindrical shape opened downward. An upper reduced-diameter section of the valve shaft 124 is fit in the center of a top wall of the valve shaft holder 132 and coupled to the valve shaft holder 132 by a push nut 133. The valve shaft 124 is fit into the center of the valve shaft holder 132 to be movable up and down. The valve shaft 124 is always urged downward by a compression coil spring 134 contracted in the valve shaft holder 132. A pressure equalizing hole 126b for equalizing pressures in the valve chamber 121 and the can 140 is formed in the sidewall of the guide bush 126.
A return spring 135, which is a cylindrical compression coil spring, is attached to the outer periphery of the push nut 133 pressed in and fixed to the upper end of the valve shaft 124. When the fixed screw section 125 of the guide bush 126 and the moving screw section 131 of the valve shaft holder 132 are unscrewed, the return screw 135 comes into contact with the inner surface of the can 140 and urges both the screw sections 125 and 131 to be screwed again.
The valve shaft holder 132 and the rotor 130 are coupled via a support ring 136. An upper projection of the valve shaft holder 132 is fit in an inner peripheral hole of the support ring 136 and caulks the outer periphery of this upper projection, whereby the rotor 130, the support ring 136, and the valve shaft holder 132 are coupled.
A ring-like lower stopper body 127 is fixed to the guide bush 126. A tabular lower stopper piece 127a is protrudingly provided in an upper part of the lower stopper body 127. A ring-like upper stopper body 137 is fixed to the valve shaft holder 132. A tabular upper stopper piece 137a is protrudingly provided in a lower part of the upper stopper body 137 to be engageable with the lower stopper piece 127a. 
The stator 142 has plural lead terminals 154 connected to the stator coils 153. A connector 156 to which plural lead wires 155 are connected is coupled to the lead terminals 154. A cover 157 that covers the connector 156 is welded to the stator 142. A filler 158 such as epoxy resin is filled in the cover 157.
The can 140 is fit in the fitting hole 144a of the jacket section 144 of the stator 142. The jacket section 144 is prevented from rotating with respect to the valve main body 120 and the can 140 by a baffle member 159 bonded to the lower surface of the jacket section 144. Although not shown in the figure, the baffle member 159 has a horizontal top wall and two arm sections extending downward from both side edges of this top wall. In the top wall, a positioning section projecting upward from one end of the top wall is formed. The baffle member 159 is fixed by inserting a projection projecting from the lower surface of the stator 142 into an attaching hole formed in the top wall and crushing the projection with an ultrasonic welder. Reference numeral 145′ denotes the crushed projection. The stator 142 fits in the fitting hole 144a over the can 140 and pushes the can 140 in the direction of the valve main body 120. Then, the two arm sections of the baffle member 159 nip and hold the pipe 120b, which extends in the horizontal direction, in a radial direction thereof. Thus, the stator 142 can be extremely easily and surely fixed to the valve main body 120 and the can 140.
In the motor-operated valve 100, the feed screw mechanism described above is usually adopted to move the valve shaft 124 smoothly. The rotation of the rotor 130 is converted into the up and down movement of the valve body 123 by the feed screw mechanism to change a valve opening to thereby control a flow rate of a fluid. Because of the structure of the feed screw mechanism, it is inevitable that a certain degree of backlash occurs in a screw engaging section of the male screw and the female screw.
A sectional view in which a part of the feed screw mechanism is enlarged is shown in FIG. 15. When the valve is opened, the moving screw section 131 is pushed down by the rotation of the rotor (the fixed screw section 125). When the fluid flows in a direction from the pipe 120b to the pipe 120a in FIG. 14, an upward force due to the pressure of the fluid is rarely generated. As a result, a load applied to the moving screw section 131 is mainly a downward load. In other words, as shown in FIG. 15A, a lower spiral surface 125b of the fixed screw section 125 comes into contact with an upper spiral surface 131a of the moving screw section 131 and pushes down the moving screw section 131 to bring the moving screw section 131 into contact with the valve seat. To open the valve from this state, first, the fixed screw section 125 has to be rotated to a state shown in FIG. 15B to absorb the backlash.
When the fluid flows in a direction from the pipe 120a to the pipe 120b in FIG. 14, a load applied to the screws is applied upward because of a force generated by the flow of the fluid. Thus, as shown in FIG. 15A, the valve is opened while the screws are in contact with each other on the upper side. The screws in contact with each other on the upper side may move to the lower side after the valve opening because of a balance of a load based on a decrease in the upward load due to the flow.
As described above, since a way of contact of the screws changes according to a difference in a flow direction, a valve opening point is different depending on a flow direction of the fluid. When screw contact surfaces change during control of a flow rate, a difference in a flow rate characteristic curve occurs. When a lift amount of the valve shaft per one pulse of a driving pulse applied to a pulse motor is large, an influence due to the backlash is relatively small. However, in the motor-operated valve of this type, for example, measures for providing a reduction mechanism between the motor and the feed screw mechanism are taken according to requests for precise flow rate control in these days. Thus, the lift amount of the valve shaft per one pulse of the driving pulse tends to be small. When the lift amount of the valve shaft per one pulse applied to the motor is extremely small, a flow rate in the beginning of the opening of the valve and a flow rate in the end of the closing do not coincide with each other, which is so-called hysteresis (a flow rate difference). This flow rate difference cannot be neglected in the flow rate control.
As a form of the existing motor-operated valve, there is a motor-operated valve that adopts bellows that form a partition wall against a coolant. A main part of an example of the motor-operated valve is shown in FIG. 16 as a sectional view. Since a structure of an upper part of the motor-operated valve may be equivalent to that shown in FIG. 14, the structure is not shown in FIG. 16. The moving screw section 131 to which power of a stepping motor is transmitted is screw-engaged with the fixed screw section 125. Bellows 160 are disposed on the outer side of the valve shaft 124. Upper ends 161 of the bellows 160 are fixed to the valve main body by caulking or soldering via a ring member 167. Lower ends 162 of the bellows 160 are fixed to a valve shaft 124. The bellows 160 prevents the coolant led into a chamber 163 communicating with the valve chamber 121 from entering a can provided in an upper part of the motor-operated valve. A ball bearing member 166 that receives a ball 165 is inserted in and fixed to the upper end of the valve shaft 124. The moving screw section 131 serving as a screw shaft comes into contact with an upper part of the ball 165. The thrust in an axial direction generated by a feed screw action by the feed screw mechanism is transmitted to the valve shaft 124 side through the ball 165 and the ball bearing member 166. An upward lifting force acts on the valve shaft 124 according to a bearing load based on an elastic force of the bellows 160 and a pressure difference between the inside and the outside of the bellows 160. Therefore, the moving screw section 131 is always pushed upward and the hysteresis due to the backlash present in the screw section is relaxed. However, the bellows 160 are expensive, have poor moldability, and require complicated assembly work such as soldering. Therefore, the bellows 160 obstruct a reduction in manufacturing cost of the motor-operated valve.
There is also proposed a motor-operated valve in which a coil spring as elastic means for removing the backlash of screws present in a feed screw mechanism for converting output rotation of a motor into an axial direction displacement of a valve body is arranged between a valve holder or a valve main body and a stem or a valve shaft. In examples of the motor-operated valve, the coil spring is arranged in a position above a valve chamber (Japanese Patent Laid-Open Publication No. 8-226564 (Patent Document 2) and Japanese Patent Laid-Open Publication No. 2005-48779 (Patent Document 3)). In this case, there is a drawback in that a dimension in the vertical direction of the motor-operated valve is large.